A powerful winter storm has spread a mix of heavy snow, sleet and freezing rain from the Ohio Valley and mid-south to New England, compounded by bitter, Arctic cold gripping much of the US east of the Rockies.
Winter storm warnings were posted for most of the eastern third of the United States covering 118 million people, as the deep freeze strained energy supplies in some areas.
The National Weather Service also predicted widespread and prolonged travel disruptions.
An estimated 157 million Americans were warned to bundle up against cold ranging from sub-zero temperatures along the Canadian border to below-freezing as far south as the Gulf of Mexico.
The Arctic blast was accompanied by gusty conditions that sent wind-chill conditions - a measure of how cold it feels based on the rate of heat loss from the body - plunging as low as -45C in the northern Plains.
Some of the heaviest snowfall, up to 30cm or more since the storm developed on Friday, was measured yesterday in parts of Colorado, Illinois, Indiana, Missouri, New Jersey, New York and Pennsylvania.
New York Governor Kathy Hochul said she had mobilised National Guard troops in New York City, Long Island and the Hudson Valley to assist with the state's emergency storm response.
The onslaught of snow, ice and winds hit air travel especially hard, with major carriers forced to cancel more than 11,000 US flights that were scheduled for yesterday, according to an industry tracking service FlightAware.com.
Ronald Reagan National Airport, located in northern Virginia just across the Potomac River from Washington, was effectively closed altogether.
Airports serving other major metropolitan areas, including New York, Philadelphia and Charlotte, North Carolina, had at least 80% of their flights cancelled yesterday, FlightAware data showed.
Power outages were widespread across the south, where freezing rain deposited layers of ice up to an inch thick, toppling tree limbs and transmission lines.
More than 1 million homes and businesses in eight states from Texas to the Carolinas were without electricity at the height of the storm, according to utility data posted online.
Tennessee bore the greatest brunt of energy cuts, accounting for about a third of all the outages.
More than 800,000 electricity customers remained without power as of 9:29pm EST (2:29am Irish time), according to PowerOutage.us.
In parts of the mid-Atlantic, heavy snow early in the day gave way to sleet and freezing rain, adding to treacherous driving conditions and making it more difficult for street crews to safely clear road surfaces.
Heavy ice was reported by the weather service to be accumulating across the interior sections of the Eastern Seaboard as far south as Atlanta, as the low pressure system driving the storm moved through the Appalachian Mountains.
Calling the storm "historic", US President Donald Trump on Saturday approved federal emergency disaster declarations for a dozen states, mostly in the mid-south.
On Saturday, 17 states and the District of Columbia declared weather emergencies.
The Department of Energy (DOE) on Saturday issued an emergency order authorising the Electric Reliability Council of Texas to deploy backup generation resources at data centers and other major facilities, aiming to limit blackouts in the state.
The DOE issued an emergency order yesterday to authorise grid operator PJM Interconnection to run "specified resources" in the mid-Atlantic region, regardless of limits due to state laws or environmental permits.
While the storm system was expected to drift away from the east coast into the Atlantic today, more Arctic air is forecast to rush in behind it, prolonging bitter cold, icy conditions over the next few days, the weather service said.
"The situation with this storm is pretty unique, just because it's going to stay cold for a period of time," Homeland Security Secretary Kristi Noem said on the Fox News Sunday Briefing programme.
"This ice that has fallen will keep those (power) lines heavy, even if they haven't gone down immediately," she added.
